The diacylglycerol lipases: structure, regulation and roles in and beyond endocannabinoid signalling

Abstract

The diacylglycerol lipases (DAGLs) hydrolyse diacylglycerol to generate 2-arachidonoylglycerol (2-AG), the most abundant ligand for the CB(1) and CB(2) cannabinoid receptors in the body. DAGL-dependent endocannabinoid signalling regulates axonal growth and guidance during development, and is required for the generation and migration of new neurons in the adult brain. At developed synapses, 2-AG released from postsynaptic terminals acts back on presynaptic CB(1) receptors to inhibit the secretion of both excitatory and inhibitory neurotransmitters, with this DAGL-dependent synaptic plasticity operating throughout the nervous system. Importantly, the DAGLs have functions that do not involve cannabinoid receptors. For example, 2-AG is the precursor of arachidonic acid in a pathway that maintains the level of this essential lipid in the brain and other organs. This pathway also drives the cyclooxygenase-dependent generation of inflammatory prostaglandins in the brain, which has recently been implicated in the degeneration of dopaminergic neurons in Parkinson's disease. Remarkably, we still know very little about the mechanisms that regulate DAGL activity-however, key insights can be gleaned by homology modelling against other α/β hydrolases and from a detailed examination of published proteomic studies and other databases. These identify a regulatory loop with a highly conserved signature motif, as well as phosphorylation and palmitoylation as post-translational mechanisms likely to regulate function.

Figure 1.

The exon structure of the Drosophila DAGL and the vertebrate DAGLα and DAGLβ are illustrated with a linear schematic. The exon boundary locations are shown with vertical lines with the boldest lines indicating their conservation across the three enzymes, and the intermediate thickness lines indicating conservation between two of the enzymes. The region encoding the 4TM domain is shown in blue, the catalytic domain in red, and the tail in grey. Within the catalytic domain a cysteine rich sequence is highlighted in yellow and the regulatory loop encoded by a solitary exon (see figure 2 for details) is coloured green. The overall domain structure of DAGLα and DAGLβ are also shown as schematics with the catalytic domain in red and the cysteine rich insert and regulatory loop again shown in yellow and green, respectively.

Figure 2.

The fold topologies of the catalytic domains of three α/β hydrolase lipases—namely (a) the pancreatic lipase, (b) hormone sensitive lipase and (c) DAGL. The fold consists of a core eight β-sheets (shown in dark blue) linked in various ways via α-helices (red), β-sheets (light blue) and loops. The relative location of the catalytic triad (serine, aspartic acid and histidine) is also shown. The hormone sensitive lipase has a large regulatory module inserted between the serine and aspartic acid (highlighted in green). The DAGLs have two conspicuous inserts, a cysteine rich insert show in yellow and a putative regulatory loop shown in green.

Figure 3.

The current DAGL phospho-map (DAGLα top and DAGLβ bottom and numbering according to human enzymes). Phosphorylation sites identified by mass-spectrometry analysis of various cells and tissues are highlighted in the catalytic domain (red) with a large cluster identified within the regulatory loop (green) close to the signature motif (blue) found in DAGLβ. Several of the phosphorylated residues in DAGLβ are conserved in DAGLα (and vice versa), and we would predict that these can also be phosphorylated. The carboxyl-terminal tail region of DAGLα has been found to be especially heavily phosphorylated.